std::ranges::size

Calculates the number of elements in t in constant time.

Given the subexpression of which t denotes the (possibly materialized) result object as E, and the type of E as T

• If T is an array of unknown bound, ranges::size(E)
• Otherwise, if T is an array type, ranges::size(E) is expression-equivalent to decay-copy  ( std::extent_v <T> ) (until C++23) auto ( std::extent_v <T> ) (since C++23)
• Otherwise, if all following conditions are satisfied, ranges::size(E) is expression-equivalent to decay-copy (t.size()) (until C++23) auto(t.size()) (since C++23) :
  • ranges::disable_sized_range < std::remove_cv_t <T>> is false
  • decay-copy (t.size()) (until C++23) auto(t.size()) (since C++23) is a valid expression of integer-like type
• Otherwise, if all following conditions are satisfied, ranges::size(E) is expression-equivalent to decay-copy (size(t)) (until C++23) auto(size(t)) (since C++23) :
  • T is a class or enumeration type.
  • ranges::disable_sized_range < std::remove_cv_t <T>> is false
  • decay-copy (size(t)) (until C++23) auto(size(t)) (since C++23) is a valid expression of integer-like type, where the meaning of size is established as if by performing argument-dependent lookup
• Otherwise, if all following conditions are satisfied, ranges::size(E) is expression-equivalent to to-unsigned-like  ( ranges::end (t) - ranges::begin (t) ) :
  • T models forward_range.
  • Given the type of ranges::begin(t) as I and the type of ranges::end(t) as S, both sized_sentinel_for <S, I> and forward_iterator <I>
  • to-unsigned-like  ( ranges::end (t) - ranges::begin (t) )
• Otherwise, ranges::size(E) is ill-formed.

Diagnosable ill-formed cases above result in substitution failure when ranges::size(E)

Customization point objects

The name ranges::size denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __size_fn

All instances of __size_fn are equal. The effects of invoking different instances of type __size_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::size

Given a set of types Args..., if std::declval <Args> ( ) meet the requirements for arguments to ranges::size above, __size_fn models

• std::invocable<__size_fn, Args...>
• std::invocable < const __size_fn, Args...>
• std::invocable <__size_fn&, Args...>
• std::invocable < const __size_fn&, Args...> .
  

Otherwise, no function call operator of __size_fn participates in overload resolution.

Notes

Whenever ranges::size(e) is valid for an expression e, the return type is integer-like

The C++20 standard requires that if the underlying size function call returns a prvalue, the return value is move-constructed from the materialized temporary object. All implementations directly return the prvalue instead. The requirement is corrected by the post-C++20 proposal P0849R8

The expression ranges::distance(e) can also be used to determine the size of a range e. Unlike ranges::size(e), ranges::distance(e) works even if e

Example

#include <iostream>
#include <ranges>
#include <type_traits>
#include <vector>
 
int main()
{
    auto v = std::vector<int>{};
    std::cout << "ranges::size(v) == " << std::ranges::size(v) << '\n';
 
    auto il = {7};     // std::initializer_list
    std::cout << "ranges::size(il) == " << std::ranges::size(il) << '\n';
 
    int array[]{4, 5}; // array has a known bound
    std::cout << "ranges::size(array) == " << std::ranges::size(array) << '\n';
 
    static_assert(std::is_signed_v<decltype(std::ranges::size(v))> == false);
}

ranges::size(v) == 0
ranges::size(il) == 1
ranges::size(array) == 2

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also